The present invention relates to an activating device for a downhole tool, and relates particularly, but not exclusively, to an activating device for controlling the activation and deactivation of an enlargement tool driven downwardly with a drill bit to enlarge a borehole through a formation to access a sub-surface reservoir of liquid and/or gaseous hydrocarbon.
An example of a downhole enlargement tool with which the activating device of the present invention may be used is an under reamer, although the activating device may be used with other downhole tools, such as a stabiliser etc.
Hole enlargement tools generally use pressure operated pistons which move outwardly from a tool body in order to displace working elements of the tool to an outwardly deployed position. The pressure used to operate the pistons comprises drilling fluid supplied under pressure to operate the drilling bit.
In some cases, with existing systems the pistons move in and out of the main body a number of times, depending upon the cycling of the tool. Various means have been used to retain the pistons, including dovetails provided on mandrels and caps retained with cap screws or bolts which in turn retain the pistons.
Some systems help to transmit torque on turning of the cutting pads of the tool in relation to the main body. Spring retention can be provided to push the pistons back into the main body when the tools are in the deactivated mode. Also, known systems have a pre-determined path of travel of the activating pistons between inward and outward limiting positions, namely an inward inoperative position and an outwardly deployed operative position. By moving between the limiting positions, there is no means of adjustment of the length of the path of travel of the pistons to provide different diameters of hole enlargement if required.
Existing systems have used shear pins to hold pistons in the tool body in the inward deactivated position. The shear pins are arranged to break as a result of fatigue from the force applied by a predetermined number of tool cycles. However, this can result in the shear pins breaking too early resulting in unwanted piston deployment.
Furthermore, in existing systems the torque applied to the cutting elements of the enlargement tool is fed back directly to the activating pistons, which is disadvantageous because it increases friction between the pistons and the bores in which the pistons are held and therefore increases wear and tear which can eventually lead to malfunction of the pistons.
Moreover, existing tools are generally constructed by mounting the pistons in their respective passages and then bolting the pistons in from the outside. The bores for the bolts provide paths for leakage.
The present invention therefore seeks to provide an improved activating device for a downhole tool, in which the extent of radial outward movement of the piston may be varied if required, to vary the outward projection of the cutting elements of the enlargement tool, and to at least reduce the frictional feedback to the pistons during rotation of the enlargement tool. The present invention also seeks to provide a tool having improved sealing against leakage with fewer sealing elements. The present invention also seeks to provide an improved method of retaining pistons in the inward deactivated position.